bad pressure switch?

flyinbryan on Mon September 13, 2010 12:25 AM

had a problem today where my a/c stopped blowing cold. the compressor was not coming on at all. (happened during an extended idle (maybe 15 mins)

i figured it was just low so i went and got one of those kits with the little gauge, unplugged the little switch and jumped across it and the compressor came on and i put the can in. at first i wasnt reading the gauge correctly and im pretty sure i over filled it. so understanding that the gauge is to be read with the compressor on, i vented freon till i was back into the proper range.

i removed the hose and little wire jumper and plugged the wires back into the evap and it came on, but then went right back off. i took a little 1/2" wrench and tapped on the pressure switch on the evap. and it came on and worked for a good while (several hours) then it quit again, so i put the little gauge back on and verified that the charge was still good (and it is still good according to the little gauge when the compressor is on) tapped on the little switch and it came back on and is still working.

is it possible that the pressure switch on the evap is going out? and can i change this switch without all the freon blowing out as soon as i remove it?

also, i added a whole can of freon, then realizing it was overfilled, released some till it was good. is there a chance i only released gas (some liquid came out but it didnt seem like enough to fill that can) and left liquid and now have it screwed up that way?

HECAT on Mon September 13, 2010 7:24 AM

Charge accuracy is measured by weight and system performance in analyzed by both high and low pressure. System may be struggling to properly remove heat after extended idle, and tripped high pressure cut out. Check engine cooling system, fan clutch, heat exchanger stack seals, stack restrictions (dirt, bugs, etc.), and condition of shroud.

Parts store "death kit" will often contain refrigerant with sealer, which is a system contaminant; and a tire pressure gauge, which is useless to determine correct charge. Venting of any refrigerant is illegal.

Since charge accuracy is unknown, I suggest a proper evacuation, vacuum, and recharge procedure be performed to verify charge level; followed by a proper system analysis, with the proper testing procedures and the reading of both high and low pressures.

Sometimes a schroeder valve is under the low pressure switch and it can be changed without system discharge.

GM Tech on Mon September 13, 2010 8:00 AM

You probably widened the contacts inside the wiring harness when you jumpered it, now it has a loose connection- be sure it is the switch and not the connection to it--- does compressor stop/start when you wiggle the connection? Yes, there is a shrader valve un der your switch, - I've never seen one on a GM without it.........

-------------------------The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

NickD on Mon September 13, 2010 9:49 AM

Brother-in-law purchased one of those kits, never opened the box, it was sealed, decided not to use it so gave it to me. I took it because I was curious.

First thing I did was to test that cheap low side only plastic gauge, with 30 psi, I have a traceable NTIS standard gauge, it read 20 psi! Now that is one hell of a huge error particularly on the low side. Don't know if your gauge is that way, but really a great way to start by overcharging your system.

Not only that, but when charging by pressures, those in the know, check both low and high side with precision gauges, proper engine rpm, and take careful measurements of the ambient temperature and relative humidity. That ripped off sheet of instructions said nothing about that.

Some kind of sealer in the cans, what kind, have no idea, guessing about 2-3 ounces of the stuff, if you dump in two cans of this crap, it is some kind of oil, that could more than double the amount of oil in your system even if that sealer was harmless. That can oil slug your compressor and wreck it at most, vastly reduce your system capacity at least, a lot less cooling.

One of the greatest mistakes a novice makes on an AC system is ass-u-m(e)-ing that the FREE-ON is low. Now if you want a reliable system, would have to drain and flush it, install a new accumulator, the correct amount of oil, and charge it properly by weight.

Correct way to test a cycling switch, that is what it is called, is to first make sure your static pressure is at least 45 psi, then put an ohmmeter across the contacts, its that simple.